LONDON (Reuters) - Britain should be able to reach a deal on reforming its relationship with the European Union which all sides can endorse, even though some of its demands are "difficult", Finland's Economy Minister Olli Rehn said on Wednesday.

British Prime Minister David Cameron wrote a letter on Tuesday setting out his four main objectives in renegotiating Britain's EU ties before a membership referendum due by the end of 2017.

"There are elements that could form a basis for an agreement ... there are elements that are more difficult to digest," Rehn, a former European Commissioner, told BBC Radio.

"Overall I think a package of elements could be agreed that could be possible to endorse both in the UK and the rest of the European Union," he added.

Rehn said Britain was "essential" in strengthening the EU's external voice, and Finland supported Cameron's push for reform in areas such as boosting competitiveness and protecting the bloc's single market.

"Definitely the European Union needs reform. That is why the UK has allies among other member states," he said.